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<title>Debye-Huckel model</title>
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    <a href="SP_0_Main.htm"><img src="images/abacus.gif" border="0" width="32" height="32" alt="Calc." title="Calc."></a>&nbsp;</td>
    <td align="left" valign="middle"><font size="-1"><a href="SP_0_Main.htm">SED &amp; PREDOM</a>&nbsp;/ <a href="SP_Ionic_Strength.htm">Ionic Strength</a></font></td>
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<p>The <b>Debye-H&uuml;ckel model</b> is based on electrostatic theory,
and it only considers long-range interactions between ions:
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	<td valign="top"><nobr>log &#947;<sub><i>i</i></sub><font size="+1"> </font>= </nobr></td>
	<td valign="top"><nobr>&#8722;<i>A Z<sub>i</sub></i><sup>2</sup></nobr>
	<nobr>&#8730;<b><i>I</i></b> &nbsp;/
	<font size="+1">&nbsp;(</font>1+<i>&aring;B<sub>&#947;</sub></i> &#8730;<b><i>I</i></b><font size="-2"> </font><font size="+1">)</font></nobr>
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<p class="cont">
where <b><i>I</i></b> is the <a href="SP_Ionic_Strength.htm">ionic strength</a>;
<nobr>&#947;<sub><i>i</i></sub></nobr> is the
<a href="SP_Ionic_Strength.htm#Act_Coef">activity coefficient</a> of an
ion &#147;<i>i</i>&#148; with electric charge <nobr><i>Z<sub>i</sub></i>;&nbsp;</nobr>
<i>A</i> and <nobr><i>B<sub>&#947;</sub></i></nobr> are solvent parameters
<nobr>(<i>A</i>=0.509</nobr> for water at <nobr>25&deg;C);</nobr>
and <i>&aring;</i> is related to the ionic size.
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<p>The Debye-H&uuml;ckel equation
is only useful in <i>very</i> dilute solutions.
For most real applications
<a href="SP_Ionic_Strength.htm#Models">activity coefficient models</a>
including short-range ion interactions must be used.
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